How do I define a required component on a Unity game object?

Question

I have a script that expected the game object to have a Terrain component.

Is there an attribute I can add (or some other way) which will prevent a designer from adding my script/component/MonoBehavior to a GameObject that does not have the Terrain component?

The only alternative I have been able to find is to attach my script to something else, and then have a public Terrain field, which requires the user to attach the terrain object to it. (since the editor won't allow you to associate a different object type.) - I just want the reverse.

Any thoughts?

Answer 1

Applying the RequireComponent decoration to a script will ensure that the GameObject has the specified component.

If the component is missing:

1. Unity will attempt to add a component of this type for you,
2. If this fails (e.g. mixing RigidBody/RigidBody2D) then the script will refuse to attach.

C# sample:

[RequireComponent (typeof (Rigidbody))]
public class PlayerScript : MonoBehaviour {
    // stuff
}

UnityScript sample:

@script RequireComponent(Rigidbody)

// stuff

Answer 2

Unity has built in functionality for this called RequireComponent. Used like this in Unityscript:

@script RequireComponent(Terrain)

This will actually automatically add a Terrain component to the object if it doesn't have one already. If the user removes the Terrain component, your script will throw a compile error about the missing component.

Answer 3

Two methods come to mind, but none are very elegant.

1) Implement OnValidate(). The downside is that's it's called only when modifying component's values, or entering/exiting game mode.

void OnValidate() {
    if (GetComponent<Terrain>() == null) {
        Debug.LogError("You can't attach this component without terrain!");
        DestroyImmediate(this);
    }
}

2) Make the script run in the editor, and implement OnAwake(). But be aware that methods like Update() will be called in the editor.

[ExecuteInEditMode]
class MyScript: MonoBehaviour {
    void OnAwake() {
        if (GetComponent<Terrain>() == null) {
            Debug.LogError("You can't attach this component without terrain!");
            DestroyImmediate(this);
        }
    }
}